1. Field of the Invention
The present invention relates to interferometers for making highly accurate measurements of wave front aberrations, particularly to phase-shifting point diffraction interferometers.
2. State of the Art
Optical metrology is the study of optical measurements. An area of optical metrology relevant to the present invention is the use of an interferometer to measure the quality of a test optic, such as a mirror or a lens. One important recent application of optical metrology is the testing of projection optics for photolithography systems. Modern photolithography systems used to fabricate integrated circuits must continually image smaller features. To do so, systems are confronted with the diffraction limit of the light employed to image a pattern provided in a reticle. To meet this challenge, photolithographic systems must employ successively shorter wavelengths. Over the history of integrated circuit fabrication technology, photolithography systems have moved from visible to ultraviolet and will eventually move to even shorter wavelengths, such as extreme ultraviolet.
As with all optical imaging systems, various aberrations such as spherical aberration, astigmatism, and coma may be present. These aberrations must be identified and removed during the fabrication and/or alignment of the projection optics, or the projection optics will introduce substantial blurring in the image projected onto the wafer.
In order to test the projection optics for various aberrations, interferometers may be employed. Conventional interferometers, based upon the Michelson design, for example, employ a single coherent light source which is split into a test wave and a reference wave. The test wave passes through the optic under test and the reference wave avoids that optic. The test and reference waves are recombined to generate an interference pattern or interferogram. Analysis of the interferogram and resultant wavefront with, for example, Zernike polynomials, indicates the presence of aberrations.
The reference wave of the interferometer should be "perfect"; that is, it should be simple and well characterized, such as a plane or spherical wave. Unfortunately, beam splitters and other optical elements through which the reference beam passes introduce some deviations from perfection. Thus, the interferogram never solely represents the condition of the test optic. It always contains some artifacts from the optical elements through which the reference wave passes. While these artifacts, in theory, can be separated from the interferogram, it is usually impossible to know that a subtraction produces a truly "clean" interferogram.
To address this problem, "point diffraction interferometers" have been developed. An example of a point diffraction interferometer is the phase-shifting point diffraction interferometer (PS/PDI) described in H. Medecki, et al., "Phase-Shifting Point Diffraction Interferometer", Optics Letters, 21(19), 1526-28 (1996), E. Tejnil, et al., "At-Wavelength Interferometry for EUV Lithography, et al, J. Vacuum Science & Tech. B, 15, 2455-2461(1997), K. A. Goldberg, et al., "Characterization of an EUV Schwarzchild Objective Using Phase-Shifting Point Diffraction Interferometry," Proceeding SPIE, 3048, 264-270 (1997), E. Tejnil, et al., "Phase-Shifting Point Diffraction Interferometry for At-Wavelength Testing of Lithographic Optics," OSA Trends in Optics and Photonics: Extreme Ultraviolet Lithography, Optical Society of America, Washington, D.C., 4, 118-123 (1996), K. A. Goldberg, "Extreme Ultraviolet Interferometry," doctoral dissertation, Dept. of Physics, Univ. of California, Berkeley (1997), and in the U.S. Pat. No. 5,835,217 "Phase-Shifting Point Diffraction Interferometer," Inventor Hector Medecki, which are all incorporated herein by reference.
The PS/PDI is a variation of the conventional point diffraction interferometer in which a transmission grating has been added to greatly improve the optical throughput of the system and add phase-shifting capability. In the PS/PDI, as illustrated in FIG. 1, the optical system 2 under test is illuminated by a spherical wave 5 that is generated by an entrance pinhole 6 in a mask 4 that is placed in the object plane of the optical system 2. To assure the quality of the spherical wave illumination, pinhole 6 is chosen to be several times smaller than the resolution limit of the optical system. Grating 8 splits the illuminating beam 5 to create the required test and reference beams 10 and 12, respectively. A PS/PDI mask 20 is placed in the image plane of the optical system 2 to block the unwanted diffracted orders generated by the grating 8 and to spatially filter the reference beam 12 using a reference pinhole 16. The test beam 10, which contains the aberrations imparted by the optical system, is largely undisturbed by the image plane mask by virtue of it passing through a window 14 in the PS/PDI mask 20 that is large relative to the point-spread function of the optical system. The test and reference beams propagate to the mixing plane where they overlap to create an interference pattern recorded on a CCD detector 18. The recorded interferogram yields information on the deviation of the test beam from the reference beam which in the ideal case is a spherical wave. The PS/PDI mask typically comprises a square shaped window and a reference pinhole to the side. The light in the interferometer will typically be of a single wavelength. The grating 8 will transmit the zeroth-order beam straight through, but will produce a small angular change to the first-order diffractions. In the image plane, the zeroth-order, and the first-order diffractions will be in different positions, as indicated by the reference pinhole and the test window in the mask 20. The zeroth-order goes to the test beam window and the first-order goes to the reference pinhole. Phase-shifting is provided by translating the grating 8 perpendicular to the rulings of the grating. Phase-shifting improves the accuracy of the system.
The phase-shifting point diffraction interferometer tends to suffer from relatively low fringe contrast which makes the signal more susceptible to noise and therefore has the potential of limiting the accuracy of the interferometry. This low contrast is due to the imbalance between the zeroth-order test beam and the first-order reference beam and the imbalance is further aggravated by the spatial filtering of the reference beam. As is apparent, there is a need for improving the fringe contrast and thus the signal-to-noise ratio.
Previous endeavors to achieve test-beam balance include, for example, increasing the size of the phase-shifting point diffraction interferometer reference pinhole. This method is not acceptable because the accuracy of the phase-shifting point diffraction interferometer improves as the reference pinhole gets smaller. An alternative method for balancing the beams involves placing an attenuating membrane in the test-beam window. This method is also not acceptable because membrane damage and contamination caused by extreme ultraviolet radiation reduces the accuracy of the phase-shifting point diffraction interferometer.
More recent PS/PDI grating enhancements have improved the interferometric fringe contrast and thus the wavefront measurement accuracy of the PS/PDI. One method relies on reversing the test- and reference-beam orders in the PS/PDI and optimizing the duty-cycle of the beam-splitting grating. This technique is described by Naulleau et. al., U.S. patent application Ser. No. 09/176,695 filed Oct. 21, 1998. This method which employs conventional diffraction amplitude grating, unfortunately, negates one of the attributes of the original configuration which is that any aberrations imparted by ruling errors in the diffraction grating are removed by virtue of the diffracted-order from the grating being the pinhole-filtered reference-beam. When high quality gratings are used, this problem is less relevant. Current state-of-the-art electron-beam lithography fabricated gratings used in the optimized contrast configuration provide an accuracy up to about .lambda..sub.EUV /200 without averaging over grating position. On the other hand, tests have demonstrated the reference wave limited accuracy of the PS/PDI to be .lambda..sub.EUV /350 or better.